The present culture of growth of megakaryocytes shows two stages of growth: stimulative and degenerative. In the stimulative stage, the entire sequence of cellular events through to the formation and release of platelets was observed. In the degenerative stage, apparently all abnormal changes of megakaryocytes reported in various thrombocytopoietic disorders were also observed in the cultures. Thus, this culture may be used as an in vitro model for studies of thrombocytopoietic disorders, especially for the idiopathic thrombocytopenic purpura, which shows both stimulative and degenerative changes in megakaryocytes.